The present invention relates to a method of compounding of multimodal polyethylene compositions and more particularly to a method of compounding a multimodal polyethylene composition comprising a low molecular weight ethylene polymer and a high molecular weight ethylene polymer.
Multimodal polyethylene compositions are well known in the art. The term “multimodal” polyethylene compositions refers to the form of the molecular weight distribution curve of the composition, i.e. the appearance of the graph of the polymer weight fraction as function of its molecular weight. When a polymer is produced in a sequential step process, utilising reactors coupled in series and using different conditions in each reactor, the different fractions produced in the different reactors will each have their own molecular weight distribution. When the molecular weight distribution curves from these fractions are superimposed into the molecular weight distribution curve for the total resulting polymer product, that curve will show two or more maxima or at least be distinctly broadened in comparison with the curves for the individual fractions. Such a polymer product, produced in two or more serial steps, is called bimodal or multimodal depending on the number of steps. However, the multimodal polyethylene compositions may alternatively be produced by physical mixing of the different components prepared separately.
It is well known in the art that when producing a polymer compound that can be used for the manufacture of different objects, the ingredients thereof, such as the different polymers, additives, etc, should be mixed intimately in order to obtain a compound as homogeneous as possible. Usually, this mixing is done in a compounding device wherein the ingredients are mixed together and the polymers and optionally some of the additives are melted so that intimately mixing can occur. The melt is then extruded into a rod, cooled and granulated. In this form the resulting compound can then be used for the manufacturing of different objects.
Although the conventional compounding tools such as tandem mixer-single screw extruders, mixer-gear pumps and twin screw extruders with or without gear pump, offering residence time in the order of 1.5 minutes or less, generally lead to acceptable quality results in compounding polymer compositions, problems are encountered when compounding multimodal polymer compositions, and more particularly multimodal polymer compositions comprising a low molecular weight ethylene polymer and a high molecular weight ethylene polymer. When compounding multimodal polymer compositions, e.g. for the manufacture of pipes, so called <<white spots>> occur in the compounded material. These white spots have a size of about 10-80 μm and consist mainly of high molecular weight polymer particles that have not been adequately compounded in the composition. In addition to being disfiguring the white spots may adversely influence the strength of the composition. Further, when compounding multimodal polymer compositions, e.g. for the production of film, gel particles with a size of about 0.01-1 mm often occur. These gel particles appear as disfiguring heterogeneities in the finished film and consist mainly of high molecular weight polymer particles that have not been adequately compounded, i.e. dispersed, in the composition. The above described white spots and gel particles are a serious problem in the polymer industry and a solution to the problem would mean the removal of an obstacle to use otherwise superior multimodal polymer compositions.
U.S. Pat. No. 6,031,027 aims to overcome this problem by providing a method of compounding a multimodal polymer composition wherein the composition is maintained at a temperature range from 10° C. below to 10° C. above the melting point of the low molecular weight polyethylene polymer during a period from 10 to 60 seconds. Such method needs a very good control of the temperature of the polymer composition and is therefore difficult to execute on the currently used industrial compounding devices.
WO 00/24821 relates to an extrusion process of particular multimodal polyethylene compositions using an extruder equipped with a gear pump. However this technology does not permit to obtain very good results with all multimodal compositions, in particular with multimodal compositions rich in low molecular weight polymer.
The present invention provides a new method of compounding multimodal polymer compositions which overcomes the above mentioned problems and which permits to obtain a very homogeneous compound which has a reduced level of white spots and/or gels and which has superior mechanical properties.